Searching the vast array of personal data in smartphones calls for a warrant.

Ever since the US Supreme Court announced in mid-January that it agreed to hear two companion cases involving warrantless searches and seizures of cell phones, the privacy world has been abuzz. Yesterday, several prominent privacy groups filed briefs telling the court why it should stop cops from making easy, no-warrant phone searches.

One case, United States v. Wurie, involves an old-fashioned flip-phone, while the other, Riley v. California, involves a modern smartphone capable of storing much more personal information both locally and in the cloud.

The issue before the court in Riley is whether an arrest alone allows a police officer to search the vast troves of data available on a person's smartphone. In David Riley's case, his phone held a potentially incriminating photo: Riley was standing next to a red Oldsmobile allegedly involved in a shooting.

Many believe Riley is shaping up to be the most important Fourth Amendment case to reach the high court since the January 2012 opinion in US v. Jones. That caseheld that tracking a vehicle with a GPS device for 28 days violated privacy rights and constituted a search, which requires a warrant.

Previously, courts have held that when officers place individuals under arrest, the police are permitted to search the suspects' "persons and effects," and the fruits of such searches are admissible evidence in courts. But modern cell phones enable access to a wealth of personal data, the large majority of which is likely unrelated to the government’s reason for arresting someone. This technological reality arguably results in the expansion of the intrusiveness of a routine search that would previously only reveal what someone is carrying on his or her physical person.

In a press release following the filing of their brief, American Library Association President Barbara Stripling said, "Today’s cell phones are much more than simple dialing systems—they are mobile libraries, holding our books, photos, banking information, favorite websites, and private conversations. The Constitution does not give law enforcement free rein to search unlawfully through our private records."

The ALA's brief argues:

If a person were arrested while carrying a key to his personal library of 6,000 books... there is no constitutional basis to search those books incident to arrest. Yet by permitting a police officer to search today's smartphone upon arrest, that is exactly what this court would be permitting. A smartphone is a portal to a person's entire electronic library; in fact, for millions of Americans, it is their primary library.

Brewster Kahle, founder and digital librarian of Internet Archive, extended the library metaphor. "Constitutional checks are placed on the search of, for instance, a personal physical library, and these checks should also apply to the comparably vast and personally sensitive stores of data held on our phones."

David Husband, one of the co-authors of EPIC's brief, adds, "Modern cellphones provide access to vast swathes of our lives, not only data that is on the phone, but data accessible from remote servers. Warrantless searches give police access to users' entire personal lives from their phones."

EPIC's brief notes that US consumers are dependent on cell phones, which function much like passwords and provide access to remote files and information that would not be available to an unauthenticated individual. The brief notes the ease of accessibility to sensitive personal information on smartphones, given that "most mobile devices...stor[e] passwords and other login information on the device so that the person in possession of the device is able to access all of the password-protective services."

The two cases are scheduled for oral argument in the court’s April sitting, beginning on April 21. We don’t anticipate a shortage of further comments from the peanut gallery.